When surface-soldering power components to a printed circuit board, such as RF power transistors for instance, it is necessary to improve the thermal conductivity of the board with the use of thermal vias. This is because the actual board material, which is often comprised of some kind of polymeric material, has very poor thermal conductivity, whereas metallic material, such as copper, is a very good conductor of heat.
At present, thermal vias are formed by drilling one or more holes in the board material, on the surface to which components are to be soldered. In the following stages of board manufacture, the inner surfaces of respective vias are coated with a thin layer of metallic material, preferably copper. It is essentially this thin copper layer on the inner surface of the holes that conducts heal through the board.
One problem with the present-day production of thermal vias is that the solder used to fasten components to the board penetrates into the holes, therewith diminishing the amount of solder that is effective in securing said components. In addition, solder droplets are formed on the opposite side of the board in relation to said components. These solder droplets are problematic, since they greatly impair the thermal contact between all underside of the board and a cooler, and must therefore be removed at a later stage in some suitable way.